Experimental study on the stimulated saturation of terahertz free electron laser

China Academy of Engineering Physics terahertz free electron laser (CAEP THz FEL, CTFEL) is the first THz FEL oscillator in China, which is jointly built by CAEP, Peking University and Tsinghua University. It is designed as a high-repetition-rate and high-duty-cycle linac-based FEL facility. This THz FEL mainly consists of a gallium arsenide (GaAs) photocathode high-voltage direct current (DC) gun, a superconducting radio frequency (RF) linac, a planar undulator, and a quasi-concentric optical resonator. The DC gun provides a high-brightness electron beam with the bunch charge of about 100 pC and the repetition rate of 54.167 MHz. The normalized emittance of the electron beam is less than 10 mu m, and the energy spread is less than 0.75%. A 2 x 4-cell superconducting RF accelerator provides an effective field gradient of about 10 MV/m and energizes the electron beam to 6-8 MeV. The beam then goes through the undulator and generates the spontaneous radiation, which is reflected back and forth in the optical resonator and then stimulated by the electron beam. The first stimulated saturation of CTFEL in the macro-pulse mode was obtained in August, 2017. In this paper, the THz spectrum is measured by a Fourier spectrometer (Bruker VERTEX 80 V). The macro-pulse energy is measured by an absolute energy meter from "Thomas Keating Instruments". The longitudinal beam length is preliminarily calculated by the auto-correlation curve from the time-domain signal of the spectrometer. The macro-pulse duration is captured by a GeGa cryogenic detector from "QMC Instrument". The measurement results indicate that the terahertz laser frequency is continuously adjustable from 2 THz to 3 THz. The macro-pulse average power is more than 10 W and the micro-pulse power is more than 0.3 MW. The single-pass gain is larger than 2.5%. This facility is now working in macro-pulse mode in the first step, also called "step one". The minimum macro-pulse duration is about 50 mu s and the maximum is about 2 ms. The macro-pulse repetition is 1 Hz or 5 Hz. The typical pulse duration and repetition rate are 1 ms and 1 Hz, respectively. In the middle of 2018, the duty cycle will upgrade to more than 10 % as "step two". And the continuous wave (CW) operation will be obtained in "step three" by the end of 2018. The spectrum adjustment range will also be expanded to cover from 1 THz to 4 THz by then. Some application experiments have been carried out on the platform of CTFEL. This facility will greatly promote the development of THz science and its applications in material science, chemistry science, biomedicine science and many other cutting-edge areas in general.